The present invention relates to a forming process for producing sharp corners in sheet metal, and to a press brake useful in carrying out such process.
Sheet metal products having sharp corners formed therein are highly desirable for particular applications, e.g., for aluminum store fronts and aluminum window frame components. Such corners are ordinarily formed by extrusion techniques necessitating heating the stock, and requiring special tooling. These techniques are only economical for projects requiring tens of thousands of square feet of extruded shapes. Large panel sheet metals and large channels cannot be handled by extrusion, nor can specialty products.
Press brakes for forming bends in sheet metal are also well known in the art. See, e.g., U.S. Pat. Nos. 4,580,434; 4,489,586, 4,489,578; 4,486,841; and 3,978,706. However, conventional press brake tooling does not produce sharp corners in sheet metal but rather form what is known in the art as "radius" bends. Illustrations of radius bends appear in Graf U.S. Pat. No. 4,580,434, FIGS. 7, 16 and 17; and Koyama et al U.S. Pat. No. 4,486,841, FIG. 9.
In some press brakes for bending sheet metal, the lower die has an indentation or well extending longitudinally along its length. See, e.g., Holtschmidt U.S. Pat. No. 4,403,498; and Swenson et al U.S. Pat. No. 3,702,558. This configuration allows the workpiece metal to flow into the well. The resulting bend is a deformed radius bend whose radius extends beyond the lines necessary to form a sharp corner in the metal. A sharp corner may subsequently be formed by grinding the excess metal contained in the extended radius to form a sharp outer corner. This technique requires several steps, is cumbersome, and is labor-intensive.
A two-step method for producing sharp corners in sheet metal is disclosed in Lawson U.S. Pat. No. 2,023,638. The first step comprises conventional forming to produce a radius bend. The second step comprises compressing the workpiece in the area of the bend with a special male die that extrudes the metal into the sharp corner of the female die (see FIGS. 4 and 5 of Lawson). The special die comprises a split, expandable die formed from a pair of displaceable jaws, each of which has a number of saw teeth which are displaced from the plane along which the workpiece is to be bent. Upon compression of the workpiece, the saw teeth cause the metal stock to flow into and completely fill the female die cavity. However, extrusion of the metal stock increases the thickness of the stock in addition to forming a sharp corner therein (see FIG. 6 of Lawson). Besides requiring multiple operations, the technique described in the Lawson patent thus produces sheet metal products having varying gauges.